I must be a numpty or something doesn't add up.

[bad boy]

Spent 3 hours in my fantastic LBS on Monday chasing a crunching noise that only happens on 25, 23 and 21 on the back all other combos are fine regardless of the front ring. It only happens when out the saddle spinning fast.

Knowing I am quite particular about these things the list is as follows:

BB removed and checked,
Cranks removed and checked,
Front Mech limit screws and positioning checked,
rear mech removed and checked,
rear mech B screw adjusted & tension tweaked
rear hanger checked and re-aligned.

At this point we tried a new wheel and NO noise !!.....so:

Spoke tension increased,
Hub out and serviced,
Casette off and checked,
Freehub inspected,
Old inner tube inbetween spoke cross over points,

wheel back on and noise still there !!.

I decided there wasnt any more I or the lbs could do so just one of those things.

Got the bike out yesterday ready for Tuesdays 10 series at Hog Hill and on the way NO noise....WTF !!. I then remembered I oiled the chain before I left, how could it of been the chain just for those certain sprockets ??.

I did check for stiff links but only a visual check running the chain backwards looking at the jockey wheels, could it be the extreme chain line on these combos accentuated a slight stiff link or something ?

Im glad its gone but am still puzzled

 

[fossyant]

Check the B screw - prevents the jockey's running too close to the sprockets, so a badly oiled chain/slightly stiff link could cause the chain to run a little rough, pulling the jockey just enough to contact the sprockets.

 

[HLaB]

My first guess would have been b screw but you said youve checked that and surely the lbs would have spotted that.
Could it be caused by frame flex? Are you standing mostly when you are those cogs?

 

[bad boy]

I think Fossy may have hit the nail on the head.

We did check the B screw in the stand, the only thing I did was oil the chain so perhaps for those gears when out the saddle the dry chain/stiff link was causing a little bounce on the jockey wheels causing them to grind on the sprocket/s.

I thought of flex and inner chain cage rub on the front but this was my first port of call and tripple checked it all and it wasn't the 'right' sound aswell.

The B screw was the shops first port of call also so maybe when spinning without to much pressure going through the cranks it was fine and as soon as I got out the saddle things I heard it ??.

I really dont know but this sounds logical based on the fact all I did after checking all that was lube the chain.

I really hope it doesnt come back I was actually going to send it back to Planet X or not use those gear combos